Electrically insulating knobs



Nov. 24, 1970 R. TESTA 3,541,882

ILECTRICALLY INSULATING KNOBS I I Filed Dec. 51, 1968 INVENTOR. Rafa/1oZ 570 BY I bf v IITTOR/VFX United States Patent US. Cl. 74553 6 UaimsABSTRACT OF THE DISCLOSURE An instrument knob assembly includes anexternal surface of electrically conducting material, which is graspedby the operator. To insure electrical isolation between this externalsurface and the metallic shaft of the instrument, which may be at anelevated electrical potential, an intermediate insulating member ispositioned between the shaft and the external knob. Where the main knobis itself in the form of a generally cylindrical main body and removableend cap, the intermediate insulating member is preferably also composedof an insulating sleeve and a detachable insulating disc. The insulatingsleeve and disc may be molded on the interior surfaces of the cylinderand removable end plate, respectively, of the knob itself.

This invention relates to an instrument knob of the type utilized toturn a rotatable shaft, which in turn is connected to an adjustableelement or assembly of the instrument. More particularly the knob of theinvention is so constructed as to provide a substantial amount ofelectrical insulation between the exterior of the knob (even though theexterior is electrically conducting itself) and the shaft (which mayintentionally or inadvertently be at an elevated electrically potential)with Which the knob is utilized.

In many different types of instruments, it is often desired to provide amanually turnable control knob, connected to a shaft which moves anadjustable element or assembly of an instrument. Since the shaft istypically metallic, it may be at an elevated electrical voltage,particularly when it is the control shaft for an electrical element orassembly (e.g., a potentiometer, a variable transformer, an electricalswitch or the like). Even when the element or assembly to which theshaft is connected contains no electrical element intended to be at suchan elevated voltage, such a shaft may still carry an electricalpotential from other parts of the instrument through variouselectrically conducting paths. Therefore, any such shaft may be at avoltage different from true ground if any part of the apparatus has oris connected to a voltage source. Thus, any electrically operated devicemay involve a certain degree of safety hazard whenever the humanoperator must manually adjust such a shaft through an electricallyconducting knob.

Although knobs, which are themselves composed entirely of electricallyinsulating material (for example, artificial resin) may be designed insuch a configuration as to substantially totally enclose the manuallyadjustable end of such a metallic shaft, such a solution to the problemhas certain disadvantages. First, since such elec- 3,541,882 PatentedNov. 24, 1970 trically insulating knobs are of relatively soft material,special provision must be made to attach these knobs firmly (as torelative rotation) to a conventional metal shaft. Thus, the knob andshaft normally have keyways or other non-symmetrical (about the axis ofthe shaft) mating surfaces to insure against slippage of the knob on theshaft. In those applications where the knob is integrally connected toan index or scale, relative adjustment of the knob on the shaft (forcalibration, for example) is made impractical. Additional knobs madewholly of insulating materials of this type are relatively unattractive.While this drawback may not be of great significance on relativelyinexpensive consumer products, it may cause serious problems inmarketing of relatively expensive apparatus, for example, scientificinstruments.

To avoid the relative cheapness in appearance of wholly plastic knobs,such plastic knobs have been coated with a metal (e.g., chrome-plated)on their exterior surface. If such metallic coated plastic knobs are tobe fastened to the shaft both firmly and adjustably, they must typicallycontain metallic elements in their interior so as to afford metal tometal fastening surfaces. The combination of a metallic coating on theoutside and metallic fastening means on the inside of such knobs usuallyresults in leakage paths of relatively poor electrically insulatingproperties, thereby rendering the knobs unsafe if the shaft carries(usually inadvertently) any substantial electrical potential.

The present invention provides a knob of this general type (that is, onehaving a body portion of plastic, with metallic fastening means insideand a metallic coating on the outside) which is so constructed as toinsure a high degree of electrical insulation between the interior andmetallic exterior of the knob. In particular, the present inventionprovides a knob of this type in which an intermediate insulating sleeveplus other constructional features insures that the metallic coating onthe outside of the knob is completely electrically insulated from theinterior metallic elements (and therefore the instrument shaft).

An object of the invention is the provision of an instrumental controlknob of the general type just discussed, which provides a high degree ofelectrical insulation between the metallic exterior and the metallicshaft to which the knob is attached.

A more specific object of the invention is the provision of such acontrol knob, including an insulating sleeve generally positionedintermediate the metallically coated exterior body of the knob and itsinterior metal parts.

A related object is the provision of such a knob in which the majorityof the parts may be molded from synthetic resins, thereby reducing thecost thereof.

Other objects, advantages and features of the invention will becomeobvious to one skilled in the art upon reading the followingspecification in conjunction with the accompanying drawing in which:

FIG. 1 is a vertical cross section through an exemplary embodiment ofthe invention, showing the knob as fully assembled and attached to theshaft;

FIG. 2 is an end elevation, as seen from the right in FIG. 1, of themain body assembly of the knob, with the end cap and attaching screwremoved;

FIG. 3 is an end elevation, as seen from the left in FIG.l, of theremovable end cap, as disassembled from the rest of the knob;

FIG. 4 is an end elevation, as seen from the left in FIG. 1, of thefully assembled knob; and

FIG. 5 is a perspective view of the split collet (also shown in FIG. 1)to attach the knob to the shaft.

In FIG. 1, a completed knob assembly is shown attached to a shaft 8,which is operatively connected in turn to the element or assembly whichis desired to be adjusted (e.g., potentiometer). The exemplary knobassembly illustrated comprises a main body subassembly 10, comprising agenerally cylindrical outer sleeve component 12. This outer componentmay optionally include (as by being intricately formed therewith) askirt portion 14, which typically would have on its visible (right-handin FIG. 1) surface 16 an index, such as an arrow, or a scale. Most ofthe cylindrical outermost surface of sleeve component 12 will typicallybe knurled, as indicated at 18.

The other component of the knob assembly which is visible during use isa removable end cap 20, comprising a flattened conical main component 22and a protruding cylindrical flange 24. This cylindrical flange is cutsubstantially through at a series of spaced radial positions, asindicated at 26 (see FIG. 3) so as to form a series of arcuateindividual flange portions 24a, 2411, etc. (four such cuts at 26 andfour such flange portions being illustrated in FIG. 3). The outerperipheral surface of the flange portions and the inner adjacent surfaceof the main body sleeve component 12 are substantially the same diameterat their meeting interface 28, so as to cause the individual flangeportions to bend slightly inwardly, thereby forming a tight pressfitbetween the end cap and main body components.

In the preferred exemplary embodiment, both the main body outer sleevecomponent 12 and the main component 22 and cylindrical flange 24 of theremovable end cap subassembly are composed of a synthetic resin, all theexternal surfaces of which have been metal plated, as indicatedgenerally at 30. For example, the sleeve component 12 of the main bodyand the component 22 and flange 24 (which typically are integrallyformed) may each be made (as by molding) from either an electroplatinggrade of acrylonitrile-butadiene-styrene (usually designated ABS) or ofthe polypropylene plastic suitable for electroplating, marketed by theShell Chemical Company. These types of artificial resin are preferablyutilized since they may be readily metal coated by available commercialtechniques. The metal coating preferably utilized is chromium, so as toprovide a relatively untarnishable, attractive appearing surface. Suchchrome plating of elements constituted of either of the two types ofartificial resins mentioned is a commercially available service renderedfor example by Madan Plastics Company of Cranford, N.J., for A.B.S.; andMiami Plating Inc., of Dayton, Ohio, and MacDermid Inc. of Waterbury,Conn., for polypropylene.

Since such metal plating of plastic parts (12, 22) will cause all oftheir surfaces to have a metallic coating, each of the two subassemblies(10, is electrically conductive, not only on the outer surfaces but alsoon their inner surfaces (32, 34, respectively). In fact when the knob isassembled, all of the metallically coated elements 1034 form onecontinuous electrical path. For this reason all of these parts (10-34)must be electrically insulated from both shaft 8 and any metal partsthat may be physically (and therefore electrically) connected to thisshaft. For this purpose the invention provides intermediate insulatingmembers between parts 10-34 and the shaft and the metallic partsconnected thereto, as will now be described.

An intermediate insulating liner or sleeve member 40, generally in theshape of a hollow cylinder, is rigidly attached to the interior surface32 of the main body or outer sleeve component 12. This intermediateinsulating liner member may be of an artificial resin (i.e., a plastic)which may conveniently be formed inside outer body component 12 by asecond molding operation (after component 12 has been molded and platedwith metal). Preferably member 40 is made of a moldable artificial resinhaving at least moderately high structural strength, in order tosuccessfully resist the force that is applied to its internal (generallycylindrical) surface 42 by the metallic collet utilized to fasten theknob to the shaft (as will hereinafter be described). As may be seen inFIG. 1, the generally cylindrical bore defined by internal surface 42starts to widen gradually near its left-hand end, as indicated at 44.The right-hand terminal portion of internal surface 42 is re-entrant, asgenerally indicated at 46 so as to form an inwardly extending flangeportion 48 of insulating member 40. The right-hand edge surface 50 ofmember 40 is generally flat, except for an upstanding cylindrical flange52, which may be formed integrally with the member 40 during the(second) molding operation.

In an analogous manner, the removable end cap 20 may have molded to itsinterior surface 34 (after the cap has itself been molded and thenplated) a generally disc shaped insulating member 60. Member includes arela tively long (in the horizontal direction in FIG. 1) cylindricalflange 62, which may be integrally formed with member 60 during the same(second) molding operation. As may best be seen in FIG. 1, the adjacentsurfaces of the upstanding cylindrical flange 52 (of member 40) and thelong cylindrical flange 62 (of member 60) are in substan tial contact,when the removable end cap 20 is positioned on the main body assembly10. Thus, when the knob is fully assembled, elements 4062 form ancontinuous generally cup-shaped insulating layer between elements 1034and the elements along the central axis of the knob (e.g., shaft 8 andthe metal elements attached thereto, about to be described). Preferablythe long cylindrical flange 62 extends into a groove 54 in theright-hand edge surface 50 of member 40, thereby forming a rabbet jointtherewith, so as to insure an effectively continuous insulation betweenthe elements inside and outside members 40 and 60 when the knob isassembled.

Between the shaft 8 and the internal surface 42 of member 40 ispositioned a generally cylindrical split collet 70, which may be made ofbrass. For most of its (left-hand) length, collet has an innercylindrical surface 72 slightly larger than the outside diameter ofshaft 8, so as to be slideable thereon when the collet is in itsunstressed condition. The outer surface 74 of the collet issubstantially cylindrical except at its left-hand end where it graduallyflares so as to increase in diameter substantially in the same manner asthe gradually widening left-hand end 44 of internal surface 42 of sleevemember 40. Because of the presence of slots 78 cut completely through(in the radial direction) collet 70 for about /3 of its length (from theleft-hand edge in FIG. 1), the left-hand part of the collet may bereadily compressed in the radial direction so as to decrease theeffective diame ter of its inner surface 72. Such compression will becaused by the wedging action between the tapered internal surface 44 ofinsulating member 40 and the similarly flaring surface 76 of collet 70,as the collet is drawn to the right in FIG. 1. To so draw the collet tothe right relative to the shaft 8 and the knob, the right-hand portion80 of collet 70 has internal screw threads 82, so as to mate with thethreads 84 on the external surface of shank portion of a binding headscrew 86.

ASSEMBLY AND OPERATION As noted during the description of the structure,the outer sleeve component 12 is separately molded (for example, of ABSor polypropylene artificial resin), and then coated with a metallic(e.g., chromium) dress surface 30. In an analogous manner the removableend cap component 22 is similarly molded and subsequently chromed. Theintermediate insulating sleeve member 40 is then molded (in a separatesecond molding operation) into the interior surface of component 12. Asis well known in such molding operations, the various contours of thegenerally inner surface 42, 44 and 46 of insulating member 40 may bereadily formed since all of the interior surface is readily accessibleto molding forms or dies. Similarly flange 52 and groove 54 may bereadily molded into the member 40 during the same operation since theyare on an easily reachable surface thereof. In a closely analogousmanner, the disc-shaped insulating member 60 and its long cylindricalflange 62 may be molded into the generally cup-shaped interior of theremovable end cap 20 (defined by its inside surface 34 and the interiorcylindrical surface of the protruding cylindrical flange 24). Thisforming of member 60 will of course be a secondstage molding operation,after the main end cap component 22 has been both molded and metalplated.

As previously noted, at least the material of the intermediateinsulating member 40 should have suflicient structural strength toresist the force applied at its left-hand internal surface 44 by thewedging action of collet 70 upon subsequent attaching of the knob toshaft 8. For this purpose, member 40 may be molded of ABS resinspecifically chosen for its structural strength. Such ABS material isavailable commercially, for example, from Uniroyal Chemical, Division ofUniroyal, Inc., Naugatuck, Conn. (A suitable electroplating grade ofpolypropylene is available from Shell Chemical Company, Scarsdale, NewYork.)

A collet 70 will be placed inside the completed main body subassembly(i.e., comprising elements 12-18, 30, 32 and 40-54) and loosely attachedthereto by mak ing a few turns on binding head screw 86 (which willenter through the aperture defined by inwardly extending radial flangeportion 48). The thus assembled main body 10, collet 70 and screw 86(the removable end cap 20 still being unattached) will then be slid ontothe end of shaft 8. Tightening of screw 86 will then draw collet 70 (tothe right in FIG. 1) along the internal surfaces 42, 44 of member 40,thereby causing tapered surface 44 to wedge the flared left-hand end 76of the collet radially inwardly toward shaft 8. Because of the splitnature of the collet (i.e., slots 78), substantially the entireleft-hand half of collet 70 will close about shaft 8 with increasingtension as screw 86 is tightened. After completion of this fixing of themain body subassembly 10 on the shaft 8, the removable end cap 20 issimply pushed onto the righthand open end of main body component 12.Since the surfaces mating at interface 28 (of the interior surface ofcomponent 12 and the outer surface of the protruding cylindrical flange24 of end cap component 22) have? slight negative clearance, end cap 20will be firmly held by friction on the right-hand end of the main bodysubassembly 10. In particular, the individual arcuate flange portions24a, 24b, etc. will be sprung inwardly slightly (as allowed by radialcuts 26) so as to form a firm friction fit between elements 12 and 22.At the same time the rabbeted or interlocking parts (52, 54 and 62) ofthe two insulating members (40 and 60) will mate so as to insurecomplete insulation of the interior metal parts (shaft 8, collet 70 andscrew 86) from the metal coated exterior knob elements (12 and 22).Although the left hand part 56 of insulating member 40 does not actuallyenclose the metal parts (shaft 8 and collet 70), there is no electricalpath between these elements and (the lefthand parts of) the main knobbody component 12, the left-hand end 56 of the insulating sleeve member40 further shielding any tendency for an electrical lea to occur in thislocation.

Insulated knobs conforming substantially to the illustrated embodimenthave provided sufficient electrical insulation between the shaft (andother internal metal parts) and the exterior parts that there was nosubstantial current leakage to the external parts (10, 20) even whenelectrical potentials in excess of 4,000 R.M.S. volts were applied toshaft 8. Specifically, the knobs passed the following rigid test. Theentire assembly was maintained at high relative humidity (83 i-3%) atroom temperature for 24 hours. The assembly was then subjected to anincreasing AC. voltage (across the shaft to the outside of the knob),starting at about 2,000 volts. The voltage was gradually increased inmoderate (about 200 volts) steps until the upper test limit of 4,000volts was reached. No breakdown or sparkover occurred, even though the4,000 volt potential was continued for over one minute and the voltagethen brought back to 2,000 volts in the same stepped manner.

Although a single exemplary embodiment of the invention (which has beenthe successfully tested in prototype form) has been disclosed in detail,it will be obvious to one skilled in the art that many changes may bemade therein without departing in principle therefrom. For this reasonthe invention is not limited to any of the details of this singleexemplary embodiment, but rather is defined solely by the scope of theappended claims.

What is claimed is:

1. An electrically insulating knob assembly having a manually graspableknob comprising a generally cylindrical main portion and a generallyflat end plate, both having an electrically conducting external surface,which knob is adapted to be rigidly attached to and partially surroundone end of a metallic shaft, comprising:

a generally cylindrical intermediate insulating sleeve,

positioned between said electrically conducting cylindrical main portionand said metallic shaft, so as to insulate at least said cylindricalmain portion from said shaft;

a generally disk-shaped insulating member, positioned between saidgenerally flat end plate and the end of said metallic shaft, and ingenerally overlying relationship to the adjacent open end of saidintermediate insulating sleeve;

means for sealingly connecting said adjacent open end of saidintermediate insulating sleeve and said diskshaped insulating member;

whereby said sleeve and said member form a generally cup-shaped,effectively continuous intermediate insulating assembly, completelyisolating all parts of the electrically conducting external surfaces ofboth said main portion and said end plate from said metallic shaft.

2. An electrically insulating knob assembly according to claim 1, inwhich:

said end plate is removably connected to said cylindrical main portionof said knob;

said intermediate insulating sleeve is rigidly attached to said mainportion;

said disk-shaped insulating member is rigidly attached to said endplate;

and said connecting means comprises releasably connectable parts of saidintermediate insulating sleeve and of said disk-shape insulating member;

whereby said end plate may be removed along with said disk-shapedinsulating member.

3. An electrically insulating knob assembly according to claim 2, inwhich:

said releasably connectable parts comprise rabbeted flanges extending,respectively, from said intermediate insulating sleeve and saiddisk-shaped insulating member;

whereby a releasable but electrically insulated connection between saidsleeve and said member is obtained.

4. An electrically insulating knob assembly according to claim 2, inwhich:

said intermediate insulating sleeve comprises synthetic resin, moldedonto the interior cylindrical surface of said main portion.

5. An electrically insulating knob assembly according member completelyisolate all parts of such electricalto claim 4, in which: 1y conductingmaterial from said metallic shaft.

said disk-shaped insulating member also comprises synthetic resin,molded onto the interior surface of said References Cited end plate. 5UNITED STATES PATENTS 6. An electrically insulating knob assemblyaccording 3 061 869 11 19 2 Scalo et 1 237 53 XR to claim 1, in which:3,313,057 4/1967 Leddy 16-121 XR said generally cylindrical main portionand said generally flat end plate both are of such construction thatFRED MATTERN, Pflmafy EXamlIler all of their surfaces compriseelectrically conducting 10 SHOEMAKER, Assistant Examiner material;

whereby said generally cylindrical intermediate insulat- US. Cl. X.R.

ing sleeve and said generally disk-shaped insulating 23753

